The present invention relates generally to an image-formation optical system and an imaging system that incorporates the same. More particularly, the invention is concerned with imaging systems such as digital still cameras, digital video cameras harnessing solid-state image pickup devices, e.g., CCDs or CMOSs, and small-format cameras, surveillance cameras that are mounted on cellar phones and personal computers.
In recent years, electronic cameras using solid-stage image pickup devices such as CCDs or CMOSs to take subject images have come into wide use in place of silver-halide cameras. For imaging systems mounted on portable computers, cellular phones, etc. of those electronic cameras, size and weight reductions are especially demanded.
Some conventional image-formation optical systems used with such imaging systems are made up of one or two lenses. With those optical systems, however, any high performance is not expectable because of their inability to correct field curvature, as already known from discussions about aberrations. To achieve high performance, therefore, it is required to use three or more lenses.
A problem with CCDs is, on the other hand, that as off-axis light beams emerging from an image-formation optical system are incident on an image plane at too large an angle, the ability of microlenses to concentrate light does not satisfactorily work, causing the brightness of an image to change noticeably between its central area and its peripheral area. For this reason, the angle of incidence of light rays on the CCD, i.e., the exit pupil position is an important design consideration. For an optical system comprising a limited number of lenses, the position of an aperture stop is of importance.
With those problems in mind, front stop triplet type image-formation lenses have been put forward, as disclosed typically in Patent Publications 1–12.
Patent Publication 1
JP-A 1-144007
Patent Publication 2
JP-A 2-191907
Patent Publication 3
JP-A 4-153612
Patent Publication 4
JP-A 5-188284
Patent Publication 5
JP-A 9-288235
Patent Publication 6
JP-A 2001-75006
Patent Publication 7
JP-A 55-73014
Patent Publication 8
JP-A 1-307712
Patent Publication 9
JP-A 4-16811
Patent Publication 10
JP-A 2001-100090
Patent Publication 11
JP-A 2001-174701
Patent Publication 12
JP-A 2001-183578
However, such prior arts have a lot of problems as described below.
With the optical systems disclosed in Patent Publications 1, 2, 3, 4 and 5, correction of coma and astigmatism due to off-axis light rays is difficult because the first positive lenses are each in a double-convex form. In any case, the half angle of view is limited to about 25°.
With the optical system of Patent Publication 6, such adverse influences as mentioned above are lessened by configuring the first positive lens in a meniscus shape convex on its image side. However, the length of the optical system is still large, failing to achieve significant size reductions.